1. Field of the Invention
The present invention relates to a matrix printed board which comprises an insulating layer and two conductive layers, i.e., a first conductive layer provided on one surface of the insulating layer and a second conductive layer provided on the other surface of the insulating layer, and a process of forming the same.
2. Related Background Art
Conventionally, manufacture of a matrix printed board uses two film-forming steps by for example evaporation and two photolithographic steps. However, this method reduces the yield and increases the manufacturing cost due to the use of a large scale manufacturing apparatus and many manufacturing steps.
There is another method which simplifies the manufacturing step by forming the first conductive layer using a thick film. This method, however, may create unevenness on the upper surface of the insulating layer formed on the thick-film conductive layer to thereby result in an improper second conductive layer pattern.
When the second conductive layers are formed by screen printing, improper conduction is likely to occur between a particular first conductive layer and an electrical connecting portion of a corresponding second conductive layer through which the second conductive layer contacts the particular first conductive layer, which will be a problem especially when a small electric current of about several hundreds of nanoamperes, for example, related to a photosensor, is handled. In such a case, it is necessary to plate with gold the surface of the first conductive layer which the electrical connecting portion of the second conductive layer contacts and to blush the contacting surface. This means an increase in the number of steps.
Conventionally in a matrix printed board of this type, the insulating layers disposed between the first and second conductive layers are formed by printing. In this case, the insulating layers are twice printed in order to prevent the occurrence of short-circuits between the first and second layers due to pinholes which may be formed in the insulating layer. However, the inclination, to the substrate, of a step portion formed at a through hole portion or the like in the insulating layer provided in order to allow for conduction between the particular first and second conductive layers becomes sharp. Especially when the second conductive layers are made of a thin film formed by evaporation or sputtering, a resist at a step portion formed by two insulating layers in the photolithographic step of forming a pattern for the second conductive layers subsequent to forming of the thin film for the second conductive layers by evaporation or sputtering becomes about three times as thick as a resist layer at portions other than at the step portion. Thus, if the quantity of exposure is adjusted to the thickness of the resist at the portions other than at the step portion, short-circuits may occur in the pattern portion for the step portion. On the other hand, if the quantity of exposure is adjusted to the thickness of the resist layer at the step portion, disconnections or open circuits may occur in the pattern portion for the portions other than at the step portion.